The automatic production of electronic assemblies typically starts with a printed circuit board (PCB) printing process, wherein an appropriate amount of solder paste is transferred to or applied at predefined locations within the PCB. These locations represent connection pads for terminals of electronic components which are mounted to the PCB by means of a surface mount placement process, e.g. a so called pick and place process carried out by an automatic placement machine. Later, the electronic components are secured by means of a soldering procedure which can be carried out e.g. within a reflow oven.
PCB printing is a process by which a viscous solder paste is deposited through aperture openings of a stencil onto a PCB. The configuration of the stencil apertures determines the basic layout of the deposits. For the printing process the stencil is aligned to the PCB and then brought in close to or in direct contact with the surface of the PCB. An angled blade, called a squeegee, is then used to drive the solder paste across the surface of the stencil at a controlled speed and force. As a result of such a print stoke the apertures on the stencil are filled with the solder paste. When the stencil is released from the PCB the resulting contents of the filled apertures are transferred to the PCB, thereby forming deposits. With one print stroke, thousands of deposits can be placed onto the PCB surface. After PCB printing the process can be repeated thousands of times with the same stencil onto further PCBs thereby creating a high throughput process.
Previously it has been estimated that between fifty to seventy percent of the total defects in PCB surface mount assembly lines are related to the PCB printing process, and that approximately thirty to fifty percent of the total manufacturing cost is due to test and rework expenses. Thus, the step of PCB printing with a stencil is considered as to be the most critical process in the PCB manufacturing process. Furthermore, a defect that occurs in the early stages of the production of electronic assemblies will propagate, causing additional rework cost at each step in the process that the PCB goes through without being detected as defective. This stresses the importance of early detection of not only obvious printing errors (e.g., extreme lack or excess of a solder paste deposit on a connection pad), but also of possible causes of other defects resulting e.g. from degradation of solder paste quality, loss of the working viscosity point, or even failures related to a printing machine with which the PCB printing process is carried out. Thus, any attempt to enhance the performance of the PCB manufacturing lines often start with the PCB printing process.
There may be a need for improving the reliability of a PCB printing process.